An aircraft in flight requires two types of power from its power plant(s). The first of these powers is in the form of thrust to propel the aircraft forward and the second is in the form of mechanical power for driving accessories such as a fuel pump, hydraulic pump and/or an electric generator. For aircraft operating at subsonic or low supersonic velocities, both thrust and accessory power is usually provided by one or more air breathing engines, such as a turbojet. A turbojet is comprised of a casing encompassing an inlet section, a compressor section, a combustor section, a turbine section, and an exhaust nozzle section arranged in flow series so as to keep the overall width of the engine to a minimum and thereby reduce the drag. Air, from the environment surrounding the engine, enters through the inlet section and into the compressor section where it is pressurized. The pressurized air then exits the compressor section and enters the combustor section where it is mixed with fuel and ignited. This hot gas mixture of air and fuel is then expanded across the turbine section and then through the exhaust nozzle section. In the turbine section a portion of the pressure energy of the hot gas is extracted from the gas and converted into mechanical power. The remaining pressure energy of the hot gas is converted in the exhaust nozzle section to thrust for propelling the aircraft forward. The mechanical power output of the turbine section is used to drive the compressor section. Additionally, the engine has an accessory gearbox to which the aircraft's accessories are operably mounted. The accessory gearbox is driven by the compressor and is mounted along the outside of the casing. The disadvantages associated with an accessory gearbox include weight penalties and increased width and drag of the engine.
For aircraft operating at high supersonic or hypersonic velocities, (i.e. a Mach number in the range of 3 to 6), alternative engine configurations like a ramjet or turboramjet can be used to provide thrust. In these alternate engine configurations as well as with the turbojet configuration, the inlet section of the engine operates as a diffuser reducing the velocity of the entering air and as a consequence increasing the air's static temperature and pressure. This diffusion is referred to, by those skilled in the art, as the ram effect and can produce air temperatures within the compressor on the order of 2500.degree. F. and similar temperatures in the casing and accessory gearbox. This temperature is above the melting temperature of metals commonly used in accessory gearboxes and other components, within the engine, such as turbine shafts and blades. Consequently, at these operating conditions a heat sink, usually a tank of some cryogen, must be provided to cool the air exiting compressor. While it is relatively easy to deliver this cooled air to the shafts and blades of the engine, delivering sufficient cooled air to the gearbox is impractical and expensive. Further, at high mach numbers the weight and drag penalties associated with gearboxes are increased.
Consequently, it has been proposed to replace the accessory gearbox with a ram air turbine drivingly coupled to the aircraft's accessories. A ram air turbine is an impulse type turbine wheel which can be interposed into the high velocity air stream surrounding the aircraft when accessory power is required. The turbine wheel extracts kinetic energy from the air stream and converts it to mechanical power. The disadvantages of the ram air turbine include high cost and the significant drag generated when interposed in the air stream. Also, the ram air turbine cannot provide cooling flow for the engine's components.
Accordingly, a need exists for a power generating system and method that can drive the accessories, provide cooling flow, not produce excessive drag and be relativey inexpensive.